The present invention relates to an acrylic acid derivative compound and a polymer liquid crystal obtained by polymerizing it.
A photopolymerizable liquid crystal monomer having a photopolymerizable functional group imparted to a liquid crystal monomer, has both a nature as a monomer and a nature as a liquid crystal. Accordingly, when a photopolymerizable liquid crystal monomer is irradiated with light in an aligned state, it undergoes polymerization while maintaining the alignment, whereby a polymer having the alignment fixed will be obtained. The polymer liquid crystal thus obtained, has an optical anisotropy attributable to the refractive index anisotropy of the liquid crystal structure. Accordingly, a special characteristic can be imparted by controlling, the liquid crystal alignment state, and an application to a retardation film or to an optical head to be used for an optical head device, is expected.
The optical head device is a device whereby light from a light source is converged on an optical disk to write information on the optical disk, or reflected light from the optical disk is received by a light-receiving element to carry out reading out of the information from the optical disk. And, the optical head to be used for the device functions as a beam splitter.
Heretofore, the following is known as a diffraction grating to be used for an optical head. For example, there is one wherein an isotropic diffraction grating is formed in the shape of a rectangular grating (relief type) on glass or plastic by a dry etching method or by an injection molding method. As another example, there is one wherein an anisotropic diffraction grating is formed on the surface of crystal showing a refractive index anisotropy, which is combined with a quarter wave plate to provide polarization selectivity.
However, when used for an optical head, with an isotropic diffraction grating, utilization efficiency of going light (light heading to an optical disk from a light source) is about 50%, and utilization efficiency of returning light (light reflected by the optical disk and heading to a light-receiving element) is about 20%. Therefore, utilization efficiency of light in the round trip is about 10% at the maximum, and thus, there has been a problem that only low efficiency is obtainable.
On the other hand, with the method wherein an anisotropic diffraction grating is formed for the surface of a flat plate of crystal showing a refractive index anisotropy such as LiNbO3 to provide polarization selectivity, thereby to obtain a high utilization efficiency of light in a round trip, the crystal having a refractive index anisotropy, itself, is expensive and can hardly be applicable to a consumer field. Further, as a method for forming a diffraction grating, a proton exchange method is common. However, in such a case, there has been a problem that protons in the proton exchange liquid are likely to diffuse into the LiNbO3 substrate, whereby it has been difficult to form a grating with fine pitches.
When a photopolymerizable liquid crystal monomer is employed and after controlling the liquid crystal monomer alignment state, it is converted to a polymer liquid crystal, it is possible to obtain a high round trip efficiency equal to crystal showing a refractive index anisotropy. For example, a method may be mentioned wherein the polymer liquid crystal is filled in the grating to attain the high efficiency. Namely, in a liquid crystal cell, of which the surface of the substrate on one side is micro-processed to have a rectangular grating, the liquid crystal monomer is aligned usually so that the long axis direction of the liquid crystal monomer molecule is in parallel with the grating, whereupon the monomer is polymerized to form a polymer liquid crystal. At that time, the grating depth is optimized by adjusting the ordinary index of the polymer liquid crystal to be consistent with the refractive index of the grating substrate, whereby a high round trip efficiency can be obtained.
Theoretically, the diffraction efficiency will be maximum when xcex/2=xcex94nxc2x7d is satisfied, where d is the grating depth, xcex94n is the refractive index anisotropy of the polymer liquid crystal, and xcex is the wavelength. And, it is possible to obtain a high level of light utilization efficiency such that the efficiency of xc2x1primary diffracted light is about 40%, and the total efficiency is about 80%.
Since the material is inexpensive, the polymer liquid crystal can be applied to the consumer field and is expected to provide an excellent optical head. As the characteristics of such an optical head, it is required to have high durability and a high round trip efficiency with a fine pitch (at a level of at most 10 xcexcm).
As a photopolymerizable liquid crystal monomer, a compound represented by the formula 2, 3 or 4 (in this specification, Ph represents an unsubstituted 1,4-phenylene group, and Cy represents an unsubstituted trans-1,4-cyclohexylene group) is, for example, known (Takatsu, Hasebe, The 106th Photopolymer Discussion Meeting Material, III-1).
CH2xe2x95x90CHCOOxe2x80x94Phxe2x80x94Cxe2x89xa1xe2x80x94Cxe2x80x94Phxe2x80x94(CH2)5Hxe2x80x83xe2x80x83Formula 2
CH2xe2x95x90CHCOOxe2x80x94Cyxe2x80x94Cyxe2x80x94(CH2)4Hxe2x80x83xe2x80x83Formula 3
CH2xe2x95x90CHCOOxe2x80x94Phxe2x80x94Cyxe2x80x94(CH2)3Hxe2x80x83xe2x80x83Formula 4
However, the compound represented by the formula 2 (hereinafter referred to also as compound 2, the same applies to other compounds) has a tolan group in its molecule and thus has had a problem that it lacks in durability. Whereas, compound 3 shows an enantiotropic nature, but the refractive index anisotropy of the monomer itself is low. And, compound 4 shows liquid crystal nature in the vicinity of room temperature, but it has had a problem that it is somewhat difficult to use, since it is a monotropic liquid crystal.
The present invention provides an acrylic acid derivative compound represented by the following formula 1 (hereinafter referred to also as compound 1) which is a photopolymerizable liquid crystal monomer having excellent durability and a low melting point Tm and showing primarily an enantiotropic nature.
Further, the present invention provides a polymer liquid crystal obtained by polymerizing the above photopolymerizable liquid crystal monomer.
CH2xe2x95x90CR1COOxe2x80x94(CH2)mxe2x80x94(O)nxe2x80x94E1xe2x80x94Xxe2x80x94(xe2x80x94E2xe2x80x94Yxe2x80x94)pxe2x80x94E3xe2x80x94OCOOxe2x80x94R2xe2x80x83xe2x80x83Formula 1
wherein the symbols have the following meanings:
R1: a hydrogen atom or a methyl group,
R2: an alkyl group,
E1, E2, E3: each independently is a 1,4-phenylene group, wherein at least one hydrogen atom may be substituted by a fluorine atom, a chlorine atom or a methyl group,
X, Y: each independently is a single bond or an oxycarbonyl group,
m: an integer of from 0 to 8,
n: 0 when m is 0, or 1 when m is at least 1, and
p: 0 or 1.